1. Field of the Invention
The present invention relates to a method for structuring a communications network based on a asynchronous transfer mode holding communications using cells of a predetermined byte length.
2. Description of the Prior Art
In recent years, ATM (Asynchronous Transfer Mode) transmission system has been extensively studied and developed as a promising basic technology for the broadband ISDN (Integrated Services Digital Network). The ATM transmission system is a technology dividing information into blocks called cells, each thereof having a fixed length of 53 bytes (5 bytes for header and 48 bytes for information field), to achieve high-speed multiplexed information transmission and exchanging. Transmission speed 2.4 Gb/s is attained by this system. The communications network based on such ATM transmission system is expected to have applicability to the basic trunk network and subscribers system in the broadband ISDN in future.
When such communications network on the basis of high speed and broadband optical transmission is structured, survivability of the network comes to have a deep significance. This is because a network failure produces a great injurious effect on the information-oriented society. As to the technology of structuring a network having the ATM transmission system as its base, great efforts are being made for standardization by international standards committees including CCITT. There are demands for a system capable of achieving restoration from failures in the communications network taking advantage of the ATM transmission system.
A characteristic-of an ATM communications network is that, even if a virtual path is previously set up in the network, a band in a link connecting one node and the other node cannot be occupied unless a cell is actually transmitted along the path. By making use of this characteristic, it becomes possible to set up alternate virtual paths for each of working virtual paths. Then, when a failure occurs, the node on the upstream side of the working virtual path can quickly achieve restoration from the failure only by switching the working virtual path to the preset alternate virtual path.
A conventional method for achieving restoration from failures will be described with reference to FIG. 1. Reference characters A, B, C, D, E, and F denote nodes and the nodes are connected by a link 4 constructed of optical fibers. Reference numeral 1, 2, and 3 denote working virtual paths, which working virtual paths 1 to 3 are accommodated by a link 4 connecting the nodes C and D, and communication is held by transmission of cells along this route. In the route passing through the nodes A and B, there are set up alternate virtual paths 5 and 6 in advance, and in the route passing through the nodes E and F, there is set up an alternate virtual path 7 in advance.
We now suppose that a failure has occurred in the link 4 between the nodes C and D, for example, as indicated by the x mark and hence a cell has become unable to be transmitted along this link. In such case, the node D on the downstream side detects the failure and sends an alarm for station to the node C on the upstream side. Upon receipt of the alarm for station, the node C connects the working virtual paths 1 and 2 to the alternate virtual paths 5 and 6 and also connects the working virtual path 3 to the alternate virtual path 7. Thus, the communication is kept up by having cells transmitted along two routes accommodating the alternate virtual paths 5 to 7.
In the above described conventional method for achieving restoration from failures, it is required that a VPI (Virtual Path Identifier) is assigned to each of the alternate virtual paths 5 to 7 in advance. Our study shows that, in a 25-node network, the link 4 constituting each alternate route is shared by about 16 alternate virtual paths of a single-link failure, and working virtual paths affected by a single-link failure are distributed to alternate virtual paths incorporated in about 5 alternate routes.
Therefore, if it is supposed that 1000 working virtual paths are accommodated by one link, 3200 alternate virtual paths may be passing through the link. In the conventional method for achieving restoration from failures, VPI numbers must be preassigned to these alternate virtual paths. However, since the number of VPIs is limited to 4096 (12 bits), only 975 virtual paths can be set up for working virtual paths. Thus, the conventional method for achieving restoration from failures involved a problem that, while one link can accommodate 4096 virtual paths, the working VPIs which can be accommodated by a link is considerably limited in number because of the necessity of pre-assignment of the alternate virtual paths.